Feat/refine markers

[jack-davison]

This PR fixes #45.

In short, it changes this (verbose, repetitive):

leaflet() %>%
  addTiles() %>%
  addPolarMarkers(
    data = polar_data,
    pollutant = "ws",
    fun = openair::windRose,
    group = "Wind Rose"
  ) %>%
  addPolarMarkers(
    data = polar_data,
    pollutant = "nox",
    fun = openair::polarPlot,
    group = "Polar Plot"
  ) %>%
  addLayersControl(
    baseGroups = c("Wind Rose", "Polar Plot")
  )

To this (shorter, cleaner):

leaflet(data = polar_data) %>%
  addTiles() %>%
  addPolarMarkers("ws",
    fun = openair::windRose,
    group = "Wind Rose"
  ) %>%
  addPolarMarkers("nox",
    fun = openair::polarPlot,
    group = "Polar Plot"
  ) %>%
  addLayersControl(
    baseGroups = c("Wind Rose", "Polar Plot")
  )

The original will still work, but the newer syntax is a lot nicer to use if all of the polar markers are originating from the same dataset.

Also applies to addPolarDiffMarkers() and addTrajPaths(). In the first instance, both "before" and "after" inherit from leaflet(), so the user should override at least one.